The following account of the prior art relates to one of the areas of application of the present application, hearing aids. Speech processing systems, such as a speech-enhancement scheme or an intelligibility improvement algorithm in a hearing aid, often introduce degradations and modifications to clean or noisy speech signals. To determine the effect of these methods on the speech intelligibility, a subjective listening test and/or an objective intelligibility measure (OIM) is needed. Such schemes have been developed in the past, cf. e.g. the articulation index (AI), the speech-intelligibility index (SII) (standardized as ANSI S3.5-1997), or the speech transmission index (STI).